Thin-film transistors which employ semiconductor formed from an oxide in a semiconductor layer have been developed so as to be applied to, for example, display devices including liquid crystal displays and various control circuits, etc. In such thin-film transistors, oxygen is supplied from the oxide film provided in contact with the semiconductor layer to the semiconductor layer, so as to adjust the electrical characteristics including the threshold voltage. However, the rate of forming the oxide film is low, thereby taking such time for the manufacture thereof.